Mass spectrometry is used for analysing substances that can be brought to the gas phase under high-vacuum conditions, i.e. under pressures generally ranging between about 10−2 and 10−6 Pa. Though the invention is not limited to this field of use, reference in the following description will therefore be made mainly to this analysis method.
The mass spectrometry is a known analytical technique applied to both the identification and analysis of known substances. The principle on which it is based is the possibility of separating a mixture of ions depending on their mass/charge (m/z) ratio generally by applying electric or magnetic fields, either static or oscillating.
There are different ways to volatilise and ionise a sample, and there are many different kinds of ion sources, such as EI (Electronic Impact) source, FAB (Fast Atom Bombardment) source, electro-spray source, MALDI (Matrix Assisted Laser Desorption and Ionisation) source. One of the most frequently used sources is the electronic impact EI source, wherein the substance of the sample either spontaneously evaporates or is already in the gas phase. A known energy electron flow hits the molecules of the sample, which are changed into positive ions by losing one or more electrons. The ions are then accelerated by an electrostatic field and directed towards the analyser.
The diagram reporting the concentration of each ion versus the mass/charge ratio is, the so-called, mass spectrum distinctive of each compound, since it is directly correlated to the chemical structure thereof as well as to the ionisation conditions, which undergoes. The instruments employed in the mass spectrometry field, known as mass spectrometers, generally comprise three main units arranged in series: an ion source to volatilise and ionise the sample, an analyser to select the ions produced by the source according to the mass/charge ratio; and a detector to detect the ions coming from the analyser. The ion source is the part of the mass spectrometer entrusted to change the molecules of the sample into ions through the ionisation phenomenon. Moreover the produced ions must be free of moving in space for measurement of the m/z ratio.
The analyser is the part of the mass spectrometer allowing for selecting the mass/charge (m/z) ratio of the ions produced by the source. Also this measurement can be carried out in many ways, however it is always requested that the ions can freely move in the spectrometer without colliding with air molecules, which is activated by providing high-vacuum conditions therein.
According to the prior art, analysers are mainly classified as magnetic analysers, Omegatron analysers (the mass selection is carried out by using a magnetic field and a RF field), quadrupole analysers, ion-trap analysers, FI-ICR (Fourier Transform Ion Cyclotron Resonance) analysers, TOF (Time of Flight) analysers, cycloidal mass analysers (the mass selection is carried out through a suitable selection of the resulting electric and magnetic field), magnetic-sector and ion-trap analysers, optic spectroscopy cross-wire analysers (measurement of the spectra either of emission or absorption light or of photons effects on the analyzed sample). In the present work the reference is made, by way of example, to the magnetic quadrupole, and ion-trap analyzers.
The magnetic analyzer comprises a bent tube immersed into a magnetic field perpendicular thereto. The magnetic field makes the ions cover a bent trajectory. The bend radius depends on the entering ions energy and on the magnetic field B. The ion exits the analyzer only if the ion trajectory corresponds to the tube bend. If the ion bends more or less than the tube bends, it collides with the tube walls being neutralized. Therefore, for each value of the magnetic field only ions having a certain m/z ratio and a certain kinetic energy pass through the analyser, while the others are removed. From the value of the magnetic field and from the kinetic energy it is possible to go back to the m/z ratio of the ion selected by the analyser. In this way the mass spectrum, which is the graph of the intensity of the ionic current detected by the detector, is obtained depending on the m/z ratio selected by the analyser. In a mass spectrum, the presence of a peak at a certain value of m/z indicates that the source is producing ions having that m/z ratio.
Another kind of analyser frequently employed in the mass spectrometry is the quadrupole analyser. Generally, a quadrupole is a device composed of four metal parallel bars. Each couple of diagonally opposed bars is electrically linked together and a RF (radio-frequency) voltage is applied between a couple of bars and the other one. A direct current voltage is then added to RF voltage. Ions oscillate during the flight among the quadrupole bars. Only the ions having a certain m/z ratio pass through the quadrupole and reach the detector for a given ratio of the two voltages: the other ions undergo instable oscillation and collide with the bars. This allows either the selection of a particular ion, or the scansion in the field of the masses by means of the voltages variation.
A further example of mass analyser consists of an ion-trap. Based on a physical principle similar to the one of the quadrupole, the ion-trap keeps all the ions within the trap and makes them selectively free upon varying of the intensity of an oscillating electric field.
The detectors generally comprise dynodes, i.e. electronic multipliers able to amplify the very feeble current produced by the ions passed through the analyser. The signals obtained in this way are subsequently transmitted to a computer able to represent, with the aid of suitable software, the amount of each ion depending on its mass, i.e. the final mass spectrum. Moreover, the use of computers allow to quickly combine the instrument parameters with the literature search in libraries of electronic format spectra, so as to automate the compounds identification according to their spectrum and to the operative conditions with which the analysis has been carried out.
With reference to FIG. 1, a mass spectrometer device of the kind based on an electronic impact source and on a quadrupole mass analyser according to the known art is schematically shown. In FIG. 1, the device is denoted as a whole with the reference numeral (11) and it comprises an entrance section (11a), an ionisation section (11b), an analysis section (11c) and a detection section (11d).
The entrance section (11a) is generally intended for being immersed in the ambient to be sampled, which generally reaches the atmospheric pressure, from which the gas to be sampled, or analyte, enters the device. To this purpose the entrance section (11a) substantially comprises a capillary tube (13) with which a heater (15) is associated. The heater, for instance, has an electric resistance wound around the capillary tube (13). As it is known, to avoid effects due to absorption/desorption along the walls of the introduction system of gas, it is advisable to make a suitable choice of the materials as well as operating at a reasonably high temperature, for instance 100° C. that further allows for avoiding gas condensation phenomena.
In accordance with a prior art embodiment, the capillary tube (13) leads to a first transition chamber (17) defined inside a corresponding flange (19), and discharged by means of a high-vacuum pump (21). The pump (21) for instance can be a turbo-molecular pump, associated through a duct (23) at a radial side door (25) and presenting the entrance axial primary door (43) associated with the casing (41) of the device.
Downstream the first transition chamber (17) a second micro-capillary tube (27), for instance having an about 20μ diameter and being about 1-2 mm long, is provided. The micro-capillary tube (27) communicates, in turn, with a second transition chamber (29), associated with the ionisation section (11b), wherein the gas to be sampled is collected downstream the micro-capillary tube (27).
In the shown example, the ionisation section (11b) comprises an electronic impact source, wherein an ionisation chamber (31) equipped with ionisation means (33), for instance ionisation filaments, is defined. Moreover, permanent magnets can be provided for increasing the source efficiency: in this way the electrons actually describe spiral trajectories so increasing the total path inside the source. Electrostatic lenses (35) are provided downstream the ionisation chamber (31) in the transition area between the ionisation chamber (33) and the following analysis section (11c). In the ionisation chamber the molecules of the sample to be analysed, which are in the gas phase, interact with an electron beam generated by an incandescent filament and accelerated through an adjustable potential. The beam energy is normally arranged between about 10 and 100 eV.
The analysis section (11c) comprises a quadrupole device (37) downstream with the detection section (11d) comprising a detector (39), for instance a faraday cup detector and/or a SEM (Second Electron Multiplier) detector or a Channeltron detector, is provided. The analysis section (11c) and the detection section (11d) are housed in the casing (41) at a pressure of generally in the order of at least 10−3 Pa, obtained through the turbo-molecular pump (21) associated through the corresponding axial primary door (43).
Calibrated leak devices are also known in the art. Devices of this kind allow to generate controlled gas flows through the membrane as well as to quantificate leakages value, by calibrating the instruments required to detect them, during tight tests. The currently used devices are substantially of two kinds: orifice leaks, or capillary, and helium permeation leaks. The first ones, also called pinholes, are generally made by laser ablation or chemical etching. Such technologies enable apertures to be manufactured with high precision and reproducibility. An example of the first kind of devices having membranes with nanoholes (holes passing through the membrane and having a nanometric size diameter) is disclosed in US Pub. No. 2006/0144120. Devices of this kind allow for generating controlled gas flows through the membrane as well as to quantificate leakages values by calibrating the instruments required to detect them during tight tests. Another example of this kind of membrane is disclosed in WO 03/049840.
The permeation leaks however have a very unstable behaviour when the temperature changes (their value varies of about 3% per centigrade grade in case of temperatures values around room temperature), long response times. They are fragile (being made of glass, they are easily breakable even when they only fall to the ground), only suitable for helium and have a single flow value. Examples of such permeation leaks are described in DE 19521275 and WO 02/03057.
Gas sampling devices based on permeation leaks are also disclosed in U.S. Pat. No. 4,008,388, US Publication No. 2002/134933, U.S. Pat. No. 4,311,669, U.S. Pat. No. 4,712,008 and WO2008/074984. Selectively permeable membranes used in the field of mass spectrometry are also disclosed in U.S. Pat. No. 4,551,624 and Maden A M et Al: “Sheet materials for use as membranes in membrane introduction mass spectrometry” Anal. Chem., Am. Chem. Soc., US vol. 68, no. 10, 15 May 1996 (1996-05-15). Pages 1805-1811, XP000588711 ISSN: 0003-2700.
Nanoholes membranes of the above first species have not to be confused with gas permeable membranes. Membranes of the first kind have holes made artificially, e.g. by laser drilling, having substantially regular cross section along the whole length of the hole and for this reason can be calibrated according to the use of the membrane. In addition, several or many practically identical holes with parallel axis can be produced on the same membrane. On the contrary, gas permeable membranes are membranes whose natural property of the material allows for permeability of a gas or a gas mixture usually at a high temperature.
As it will be easily appreciated from the preceding description of a gas analyser according to the known art, the entrance section and the ionisation section are considerably complex both for the number of the components and for the fact that such components must be high-vacuum tight associated with each other, resulting in high costs. Moreover, the prior art devices must be equipped with vacuum pumps having considerable flow capacities as they have to absorb the flow entering the ionisation chamber, which is generally high.
An object of the invention is therefore to provide a simplified sampling gas device, which can be used in numerous applications and, in particular, associated with a gas analyser.
A further object of the invention is to provide a gas sampling device, which can be industrially produced with favourable costs.
These and other objects are achieved with the gas sampling device as claimed in the appended claims.